This invention relates to implantable stimulators generally and more particularly to implantable cardioverters and defibrillators.
Over the past 20 years, there has been substantial work toward developing a practical, implantable defibrillator. However, several significant problems still remain. Early conceptions of implantable defibrillators, such as disclosed in U.S. Pat. No. RE 27,652 by Mirowski et al., envision a system employing a ventricular endocardial electrode and a plate electrode, mounted directly to the heart, subcutaneously, or applied to the skin. However, it was recognized early on that a totally transvenous system would be desirable in order to simplify the use of implantable defibrillators. One such system is suggested in U.S. Pat. No. 3,942,536 by Mirowski et al., which discloses a transvenous lead having electrodes intended for location in the right ventricular apex and superior vena cava. Such systems were eventually tested in human beings with some success. However, currently available commercial versions of implantable defibrillators generally employ epicardial patch electrodes alone or in conjunction with transvenous electrodes.
While systems employing epicardial patch electrodes are workable, a thoracotomy is required in order to apply the epicardial electrode or electrodes. It is generally believed that it would be highly desirable to produce an implantable defibrillator which would entirely avoid the necessity of a thoracotomy, and there has been substantial work directed toward such systems, as disclosed in U.S. Pat. No. 4,727,877 issued to Kallok and U.S. Pat. No. 4,708,145 issued to Tacker et al. Both Tacker et al. and the Kallok patents disclose the use of a transvenous, two-electrode lead in combination with a subcutaneous patch electrode.
U.S. Pat. No. 4,392,407 issued to Williams et al. and co-pending, commonly assigned application Ser. Nos. 284,957 by Mehra and 284,955 by Bardy, both filed Dec. 15, 1988 disclose multiple electrode systems employing subcutaneous patch electrodes, coronary sinus/great vein electrodes, and ventricular endocardial electrodes. These electrode systems and other multiple electrode systems employing endocardial electrodes alone or in conjunction with subcutaneous electrodes appear to hold significant promise.
Where there are electrical conductors there is the possibility of electrical malfunction. In the context of pacing leads, these malfunctions have often taken the form of open circuits or short circuits, and monitoring systems have been developed to detect and remedy these problems. U.S. Pat. No. 4,140,131 issued to Dutcher, incorporated herein by reference in its entirety, discloses a pacemaker which ascertains the presence of short circuits or open circuits by measuring the impedance between the pacing electrodes and determining whether the measured impedance falls outside a predetermined range. If the measured impedance falls outside this range, a warning signal is communicated to the patient in whom the pacer is implanted by means of electrical stimulation of the tissue adjacent the pacer. A more recent example of a pacemaker which measures impedance is disclosed in U.S. Pat. No. 4,899,750 issued to Ekwall, and incorporated herein by reference in its entirety. In this pacer, measurements of impedance are stored in a log for later review by the physician to allow diagnosis of lead related problems. In some pacemakers, lead configuration is programmable between unipolar and bipolar configurations. This feature raises the possibility that the pacer may be programmed to a configuration incompatible with the leads actually implanted. The pacer disclosed in published EPO Patent Application No. 338,363, also incorporated herein by reference in its entirety, addresses the problem of inappropriate lead configuration programming by measuring impedance between pacing electrodes whenever reprogramming has taken place and reconfigures the programming of the lead configuration if the measured impedance indicates that the expected lead system is not present. The pacer also measures impedance in response to a failure to capture or other circumstances indicative of lead malfunction and reprograms the lead configuration in response to a measured impedance indicative of an electrical fault. Lead configurations are programmed and tested until a configuration exhibiting appropriate values of measured impedance is selected.